Feed forward carbon black reactor control

ABSTRACT

A carbon black process control is described wherein in the preferred embodiment responsive to a determination of the humidity or water content in the free oxygen-containing gas flowing into the carbon black reactor two separate modes of control are calculated and a selection between the modes is made depending upon the influence each of the modes has on further properties of the overall operation. For example, the iodine number of the carbon black is controlled by either manipulating the air flow or the oil flow into the carbon black reactor to maintain the iodine number constant despite changes in the humidity and the selection between the two control modes (air flow control or oil flow control) is made in accordance with a determination of the respective conversion achieved.

This invention relates to the automatic control of a carbon blackreactor responsive to a determination of the humidity or water contentof the free oxygen-containing gas utilized in the carbon blackproduction. The invention specifically relates to both a process and anapparatus involving such a control system.

BACKGROUND OF THE INVENTION

The production of carbon black under controlled conditions has becomeincreasingly important in recent years. The more constant the individualparameters of the carbon black process are kept, the more reliable andpredictable the carbon black properties will be. Unfortunately, avariety of carbon black process parameter variations occur which are notreadily controllable economically. One of the most significant of suchparameter variations is the water content of the free oxygen-containinggas such as air used during the fuel combustion for heating the carbonblack feedstock to carbon black forming temperature. The water contentof the air, which can also be expressed as the relative humidity of theair at a given temperature, has a significant influence on suchimportant carbon black process parameters as conversion, productionrate, iodine number, etc. Although it is possible to operate a carbonblack plant with air of controlled water content, the requiredpretreatment of the air or the other free oxygen-containing gas utilizedis frequently economically prohibitive.

It has been proposed in the prior art to control the quality of thecarbon black by manipulating the hydrocarbon feed rate responsive to thewater content in the free oxygen-containing gas stream. Whereas thisproposal has been a significant contribution to the carbon black art, itwould be desirable to have available a more flexible control system thatallows further optimization of the carbon black process withoutsacrificing quality control of the carbon black produced.

THE INVENTION

It is thus one object of this invention to provide a control system fora carbon black production process permitting an optimization ofconversion.

Another object of this invention is to provide a carbon black processutilizing such a control system.

Still a further object of this invention is to provide an apparatus forcarrying out the process.

These and other objects, details, features and embodiments of thisinvention will become apparent to those skilled in the art from thefollowing detailed description of the invention, the appended claims anddrawings in which

FIG. 1 shows a schematic diagram of a carbon black reactor and thecontrol system of this invention,

FIGS. 2-4 show diagrams of the functional relationship between variouscarbon black process parameters.

In accordance with this invention a control system for a carbon blackproduction operation is provided which calculates responsive to thedetermination of the water content of the free oxygen-containing gasused in the process, at least two process parameters that can bemanipulated, and the effect of the manipulation of each of theseparameters on conversion. The control system then automatically selectsthat parameter of the carbon black process to be manipulated which, forthe corresponding conversion calculation, would result in the largestconversion. The calculation of the process parameters responsive to thehumidity for water content determination is carried out such that apreselected carbon black property will remain essentially unchangeddespite changes in the water content of the free oxygen-containing gas.

More specifically, this invention in accordance with a first embodimentresides in a process to produce carbon black by the decomposition of ahydrocarbon feed heated to carbon black formation temperature. Suchheating is achieved by the combustion of fuel and a freeoxygen-containing gas. In the process of this invention, the content ofwater in the free oxygen-containing gas is automatically determined. Atleast two control signals, each being representative of one of at leasttwo process parameters which can be manipulated, are then generatedresponsive to the determination of the water content. The controlsignals are generated such that the respective manipulation of acorresponding process parameter would counteract the effect any changein the water content as determined would have on a given first carbonblack property to keep such first carbon black property essentiallyconstant. A value is then automatically calculated for a further carbonblack property for each of said at least two process parameters.Responsive to the results of this calculation, at least one of said atleast two control signals is selected for actual control. The processparameter (or parameters) is then manipulated responsive to the thusselected respective control signal (or signals). This control methodallows the maintenance of one carbon black property constant whileoptimizing another carbon black property. Thus, it is for instancepossible to maintain the iodine number constant under changing humidityconditions of the free oxygen-containing gas while manipulating eitherthe oil-feed rate or the air-feed rate and to thereby achieve thehighest conversion.

The at least two process parameters that can be manipulated preferablyare the feed rate of the hydrocarbon feed and the flow rate of the freeoxygen-containing gas or both.

The carbon black properties that can be maintained constant orrespectively optimized in accordance with this invention include iodinenumber, surface area, tint, conversion, CTAB, DBP, transmission, andproduction rate. In the preferred embodiment, the iodine number of thecarbon black produced is maintained at a constant value whereas thecontrol is selected such as to achieve the highest conversion rate.

More specifically and preferably, a humidity signal is automaticallygenerated representative of the quantity of water contained in the freeoxygen-containing gas which is introduced into the carbon black reactor.This humidity signal is then automatically converted into an oxygencontrol signal and into a feed control signal. The oxygen control signalis representative of the manipulation of the flow of freeoxygen-containing gas required to maintain the selected carbon blackproperty essentially unchanged. Similarly, the feed control signal isrepresentative of the manipulation of the flow of the feed, such as theoil-feed flow rate, required to maintain the carbon black propertiesessentially unchanged despite a possible variation in the water contentof the free oxygen-containing gas. From the oxygen control signal andthe feed control signal, a switch signal is automatically generated.This signal is representative of a comparison of two values of a furtherprocess parameter such as the conversion rate. These two values havebeen calculated automatically from the feed rate of the freeoxygen-containing gas (as represented by the oxygen control signal) andfrom the feed flow rate (as represented by the feed control signal).Depending upon which of the two values is the more desirable one for therespective process parameter, the switch signal will be such that itswitches to the control of either the flow rate of the freeoxygen-containing gas or the flow rate of the feed.

It has to be emphasized, that the process of this invention ispreferably applied to a computer controlled carbon black productionprocess. In a computer controlled carbon black production process, avariety of process parameters such as the water content of the freeoxygen-containing gas, the feed rate of this gas and its inlettemperature, the feed rate of the hydrocarbon feedstock as well as itsinlet temperature, the composition of the feed stream, the position ofthe feed nozzle within the reactor, the quench rate, the pressure dropacross the carbon black reactor (i.e., primarily the inlet pressure ofthe free oxygen-containing gas), flame temperature and others aremeasured and entered into the computer either manually or automatically.From such parameters of the process, various control operations arecomputed and carried out. This invention modifies this general computercontrol by providing a system which selects automatically one of aplurality, usually one of two, possible control modes.

Whereas the invention is applicable to a variety of carbon blackprocesses and carbon black reactors, it is presently preferred to usethe invention in connection with a carbon black process which uses anoil as the hydrocarbon feedstock and air as the free oxygen-containinggas. In some instances, particularly when larger reactors are used,oxygen enriched air can be the preferred free oxygen-containing gas inthe process.

In accordance with a further embodiment of this invention, an apparatusis provided for producing carbon black and for carrying out the controlprocess described. This apparatus comprises a carbon black reactor, ahumidity signal generator and an automatic double control unit. Thecarbon black reactor comprises essentially a cylindrically shapedhousing essentially in an axial direction, means for injecting hotcombustion gasses obtained by the combustion of fuel and freeoxygen-containing gas tangentially into the housing as well as means forwithdrawing carbon black containing smoke from the housing. The humiditysignal generator is designed to determine automatically the watercontent in the free oxygen-containing gas and generating a signalrepresentative thereof. The automatic double control unit is operativelyconnected to the humidity signal generator and the carbon black reactor.The double control unit receives the humidity signal plus signals ofother process parameters from the carbon black reactor. The unit iscapable of converting this humidity signal into two control signals;namely, a feed-control signal and an oxygen-control signal. Each ofthese control signals is computed automatically by the control unit suchas to allow manipulation of feed flow or, respectively, flow of freeoxygen-containing gas responsive to those control signals in such a wayas to keep one carbon black property essentially constant and unchangeddespite any changes in the humidity or water content of the freeoxygen-containing gas flowing into the carbon black reactor. The doublecontrol unit is equipped with a selected means capable of selecting thatone of the control signals for the actual manipulation of the respectiveflow rate which results in a larger conversion of the hydrocarbon feedto carbon black.

A carbon black reactor, together with the main elements of the controlsystem of this invention, is shown in FIG. 1. A hydrocarbon feed isintroduced into a carbon black reactor 1 via line 2. Fuel is introducedvia line 3 and free oxygen-containing gas, such as air, via line 4. Fueland free oxygen-containing gas are combusted to form hot combustiongases surrounding the axial stream of hydrocarbon feed in vortex typeflow. The carbon black containing smoke is withdrawn from the reactor 1via line 5.

The flow rate of the hydrocarbon feedstock in conduit 2 can bemanipulated by valve 21 and the manipulation of valve 21 can becontrolled by controller 22. Similarly, the flow of freeoxygen-containing gas can be manipulated by valve 41 which can becontrolled by controller 42.

The water content of the free oxygen-containing gas entering the carbonblack reactor via line 4 is determined at the location 6. The humiditysignal representative of the water content in the free oxygen gas inline 4 is entered into a computer 7. This computer 7 can be a digital oran analog computer but is preferably a digital computer. The computer 7may encompass controllers 22 and 42 as parts of the computationprograms. The units described in connection with computer 7 are portionsof computer programs but may also be hardward elements such as specialamplifiers in an analog computer or flip flops in a digital computer.

In an iodine model 71, the computer 7 calculates signals ΔAIR, 74, andΔOIL, 75, based on the humidity signal 6, and iodine number set point,72, and various other process parameters entered into the computer via73. The humidity signal, 6, is automatically compared to a referencehumidity within the iodine number model, 71. Therefore, the signalsΔAIR, 74, and ΔOIL, 75, are representative of changes in air rate or oilrate required to maintain the iodine number of the carbon black at adesired level despite changes in humidity detected by sensor 6 withrespect to the reference humidity. Whether the ΔAIR signal 74 or ΔOILsignal 75 will actually be used in the respective air controller 42 oroil controller 22 is determined by a conversion model 76 and switch unit77. The conversion model 76 calculates the change in conversion whichwould be caused by the ΔAIR signal 74 to generate a conversion changesignal 761. Similarly, the conversion model 76 calculates a conversionchange signal 762 representative of the changes in conversion whichwould be caused by the ΔOIL signal 75. The switch unit 77 is ahigh-select means which, dependent upon which one of the two signals 761and 762 is the greater, provides a change in set-point to the respectivecontroller 22 or 42 by allowing the ΔAIR signal, 74, or the ΔOIL signal,75, to be added to the set-points 24 or 44 respectively. The air rateset-point 24 and the oil rate set-point 44 may be produced in anotherpart of a computer control system or manually input by the operator.Thus switch unit 77 selects whether the air rate controller set-point 23or the oil rate controller set-point 43 is to be changed to correct forthe effect of a change in humidity. Thus, depending upon the calculationof the conversion model 76, the switches 78 and 79 will be in an OFF orin an ON position. When switch 78 is off, switch 79 is on and viceversa. Thus, in this preferred embodiment of the invention, either thehydrocarbon feed-rate or the air-flow rate is manipulated responsive tothe humidity determination while the other of the two process parametersis not manipulated responsive to the humidity determination but can bemanipulated responsive to other control operations.

In FIGS. 2-4, typical examples of the functional relationship betweenchanges of carbon black process parameters are shown. These figuresdemonstrate the advantages of this invention. Some attempts havepreviously been made to try to control a carbon black property, such asiodine number, by measuring a property of the smoke, such as methaneconcentration. The iodine number is shown in this and all the otherfigures in units determined by ASTM method D1510-70. Conversion in theseand all other figures is expressed in units of pounds of carbon blackproduced per gallon of oil. Methane concentration is expressed in volumepercent. These figures show two significant facts. FIG. 2 shows that forchanges in humidity, the iodine number changes significantly but thereis no change in methane concentration; therefore, this variable cannotbe used as a control variable. Secondly, FIG. 3 shows that while eitherair rate or oil rate may be manipulated to control iodine number, theireffect on conversion is different. Therefore, by selecting the propervariable, a higher conversion may be obtained.

Thus, in FIG. 2, it can be seen that the unit change of humidity shownabove causes the iodine number to change from 91.5 to 94.9 for adecrease in humidity and from 91.5 to 88.1 for an increase in humiditywhereas the methane content in the off-gas remains essentiallyunchanged.

A change in air flow rate by the unit given above causes the iodinenumber to change from 91.5 to 99.4 for an increase in air flow and from91.5 to 83.5 for a decrease in air flow. The methane content for anincrease in air flow changes from 0.471 to 0.412 whereas the methanecontent for a decrease in air flow changes from 0.471 to 0.530.Similarly, the unit change for the oil flow given above causes theiodine number of the carbon black to change from 91.5 to 86.8 for thecase of an increase in oil flow whereas a decrease in oil flow causesthe iodine number to change from 91.5 to 96.2. The methane content inthe case of increased oil flow changes from 0.471 to 0.510 whereas thedecrease in oil flow causes the methane content to change from 0.471 to0.432.

FIG. 3 in a similar manner shows the effect of the change of the threeparameters humidity, air flow and oil flow on the conversion rate(pound/gallon) and the methane content.

FIG. 4 shows the effect the unit changes of the three parameters oilrate, air rate and humidity have on conversion and iodine number. Itshould be noted that the diagram of FIG. 4 does not contain anyadditional information over the diagrams of FIGS. 2 and 3 but rathershows part of the respective informations in a further diagrammaticrepresentation.

The vectors in FIGS. 2-4 are each three standard deviations of thatvariable in length (30). This represents the normal variation in thesevariables observed. For normal carbon black reactors, without computercontrol, variations for the three parameters are

                                      TABLE 1                                     __________________________________________________________________________    Parameter relationship for a typical carbon black plant                                    Δair                                                                          = ±5610 SCFH                                                         Δoil                                                                          = 8.58 GPH                                                              Δhumidity                                                                     = ±(relative) 1.7%                                                  Ref.                                                                              +ΔAir                                                                       -ΔAir                                                                       +ΔOil                                                                       -ΔOil                                                                       +ΔHum.                                                                       -ΔHum.                             __________________________________________________________________________    Iodine Number                                                                             91.5                                                                              99.4                                                                              83.5                                                                              86.8                                                                              96.2                                                                              88.1 94.9                                     Conversion (lbs/gal)                                                                      4.69                                                                              4.594                                                                             4.786                                                                             4.726                                                                             4.654                                                                             4.562                                                                              4.818                                    Methane (Vol. %)                                                                          .471                                                                              .412                                                                              .530                                                                              .510                                                                              .432                                                                              .471 .471                                     __________________________________________________________________________

In the simplest application of the process of this invention, theinterrelationship between the above mentioned process parameters of thecarbon black process is approximated by a linear relationship. Theselinear models are generally always of the same form. In particular, thecoefficients for the important variables always have the same sign andmagnitude. Simple models for iodine number and conversion would be##EQU1##

In these equations, the terms used to characterize the process variablesare as follows:

I2Nφ--Iodine Number (property of carbon black)

XYCC--Yield, lb. carbon black/gallon oil

FAIR--Dry air rate, MCFH

FφIL--Oil flow rate, gal/hr.

AH20--Water flow rate in air, MCFH

Xi--Other independent variables cut-off spray position, flametemperature, make-oil spray position, etc.

The coefficients A₀ -A₃ and b₀ -b₃ may be empirically determined byconducting a designed experimental test. The exact form of the modelwill vary depending on the test design. The models shown above containonly those variables in the control system. Typical values for thesecoefficients are:

    ______________________________________                                        A.sub.0 = -3.0       b.sub.0 = 7.0                                            A.sub.1 = 1.4        b.sub.1 = -0.02                                          A.sub.2 = -0.5       b.sub.2 = 0.004                                          A.sub.3 = -2.0       b.sub.3 = -0.08                                          ______________________________________                                    

Assuming that a change in humidity (AH2φ) is counteracted in order tomaintain the iodine number by manipulating only the dry air rate FAIR,the following relationship exists:

    ΔI2NO=a.sub.1 ΔFAIR+a.sub.3 ΔAH2φ=0

or

    ΔFAIR=-(a.sub.3 /a.sub.1)ΔAH2φ

    ΔI2Nφ--change in iodine number

    ΔFAIR--change in dry air flow rate, MCFH

    ΔAH2φ--change in water flow rate in air (MCFH)

Similarly, if only the oil flow rate is changed to hold the iodinenumber constant the following relationship exists:

    ΔI2NO=a.sub.2 ΔFφIL+a.sub.3 ΔAH2φ=0

or

    ΔFφIL =-(a.sub.3 /a.sub.2)ΔAH2φ

    ΔFφIL =change in oil flow rate, gal/hr

The conversion model puts the above process parameters into a linearrelationship to calculate the conversion of oil to carbon black.

In the conversion model of the computer will determine the effect of anair rate change on the conversion as well as the effect of an oil ratechange on the conversion. The results of this determination will be:

I. Air Rate

    ΔXYCC.sub.AIR =b.sub.1 ΔFAIR

II. Oil Rate

    ΔXYCC.sub.φIL =b.sub.2 ΔFφIL

Depending upon which of the two conversion changes is larger, thecomputer will select either ΔFAIR or ΔFφIL for the control signal. Thiscan also be expressed as follows:

If (XYCC_(AIR).GT.XYCC_(OIL)) switch (1)=1. and switch (2)=0.

If (XYCC_(AIR).LT.XYCC_(OIL)) switch (2)=1. and switch (1)=0.

In the above representation, switch=(1) means that the change in therespective set point equals the change signal ΔFAIR or ΔFOILrespectively. Switch=0 means that there will be no change in therespective set point, i.e., the set point of the air flow orrespectively the oil flow will remain as prior to the control cycle. GTmeans greater than and LT means less than.

Reasonable variations and modifications which will become apparent tothose skilled in the art can be in this invention without departing fromthe spirit and scope thereof.

We claim:
 1. A process to produce carbon black by decomposition of ahydrocarbon feed heated to carbon black formation temperatures by theheat developed by the combustion of a fuel and a free oxygen-containinggas, said process comprisinga. determining the content of water in thefree oxygen-containing gas, b. generating at least two control signals,each representative of one of at least two process parameters that canbe manipulated, with the proviso that these control signals aregenerated so that the respective manipulation of the correspondingprocess parameter would counteract the effect any change in said watercontent would have on a given first carbon black property or firstcarbon black process property, c. automatically calculating a value fora further carbon black property or further carbon black process propertyfor each of said at least two process parameters, d. responsive to theresults of the calculation in step c selecting at least one of said atleast two control signals for actual control, e. manipulating theprocess parameter or parameters responsive to the so selected respectivecontrol signal or signals such as to maintain said given first carbonblack property or respectively first carbon black process propertyessentially constant.
 2. A process in accordance with claim 1 whereinsaid at least two process parameters are the hydrocarbon feed rate andthe flow rate of the free oxygen-containing gas.
 3. A process inaccordance with claim 1 wherein said first carbon black property isselected from the group consisting of iodine number, nitrogen surfacearea, tint, CTAB, DBP, and transmission and wherein said first carbonblack process property is selected from conversion and production rate.4. A process in accordance with claim 3 wherein said further carbonblack property or respectively further carbon black process property isa property different from said firsst carbon black property orrespectively first carbon black process property, and wherein saidfurther carbon black property is selected from the group consisting ofiodine number, surface area, tint, CTAB, DBP, and transmission, andwherein said further carbon black process property is selected fromconversion and production rate.
 5. A process in accordance with claim 1comprisinga. automatically generating a humidity signal representativeof the quantity of water contained in the free oxygen-containing gasentering the carbon black reactor for combustion of said fuel, b.automatically converting this humidity signal into an oxygen controlsignal and into a feed control signal, said oxygen control signal beingrepresentative of the manipulation of the flow of free oxygen-containinggas required to leave said first carbon black property or respectivelysaid first carbon black process property essentially unchanged, saidfeed control signal being representative of the manipulation of the feedflow rate required to leave said first carbon black property orrespectively said first carbon black process property essentiallyunchanged, both despite a change in humidity of the freeoxygen-containing gas, c. automatically generating a switch signal fromsaid oxygen control signal and said feed control signal, said switchsignal being representative of a first conversion as compared to asecond conversion, the first conversion being calculated on the basis ofthe oxygen control signal and the flow of free oxygen comprising gasthis signal represents, said second control signal being calculated onthe basis of said feed control signal and the feed flow rate itrepresents, d. upon the result of this calculation in step c utilizingthe switch signal to allow either an oxygen controller to manipulate theflow of free oxygen-containing gas responsive to the oxygen controlsignal or respectively to allow a feed controller to manipulate the flowof feed responsive to the feed control signal.
 6. A process inaccordance with claim 1 comprisinga. using an oil as said hydrocarbonfeed and air as said free oxygen-containing gas, b. automaticallydetermining the change in air flow rate and the change oil flow ratenecessary to maintain said first carbon black property or respectivelysaid first carbon black process property despite a change in the watercontent of the air, c. automatically calculating two conversion valuesfrom the respective air flow rate and oil flow rate, d. automaticallyselecting the change in air flow rate or respectively the change in oilflow rate for the control of the air flow rate or the oil flow ratedepending upon which of the two conversion values resulted in the highervalue, e. changing the feed flow rate or the air flow rate responsive tothe corresponding change selected.